eyespot

A light-sensitive organelle present in
unicellular organisms, green algae, zoospores,
and some gametes. Eyespots can also be found
on more complex animals that have well-developed eyes. A dragonfly, for
example, has an eyespot on each side of its head in addition to its compound
eyes.

An eyespot enables a creature to respond to changes in light and dark but
cannot gather enough information to form an image.